What can you do when a storm knocks out power in your neighborhood? Many people are prepared; they fire up a portable gasoline-powered generator. That’s great as long as fuel is available. But when it’s a widespread disaster like Hurricanes Sandy and Katrina, fuel supplies may be cut off. Disaster relief agencies can’t rely on a steady fuel supply to run their generators. Military personnel face this situation every day. It’s costly and dangerous to get fuel to the front lines, so supplies are uncertain at best. X3 Energy has a solution: the X3 series, a set of portable energy generation units that use renewable energy as their primary source while providing fuel-based generators as a backup. While both of these units are designed for commercial, industrial, emergency, or military markets, the design philosophy - renewables, batteries, and an engine generator - offers a lesson to anyone designing an off-grid energy system.

Calling these units “portable” might be a stretch. The smaller unit, the X3-Mobile 50, fits in the back of a pickup truck or a large SUV. Its big brother, the X3-20, needs to be moved by a flatbed truck. I think “transportable” is a more accurate description. Still, they meet the needs of a few important, albeit narrow, markets. Let’s check them out...

The X3-Mobile 50 (pictured above) contains a 5 kW wind turbine (10 kW optional), a 5 kW solar array that can be expanded to 20 kW, a set of rechargeable batteries, a “smart” inverter/charge controller, four 1000W metal halide lamps, and an optional engine generator. It’s capable of delivering 12V or 24V DC, and 120V, 208V, 240V, and 480V AC,.single phase or three-phase, at 50 or 60 Hz. In locations that have internet access, the X3-Mobile 50 can report real-time power status. X3 Energy reports that the battery bank can provide about 72 hours worth of energy.

Its wind turbine is mounted to a self-deploying hydraulic mast. The turbine can generate energy at wind speeds as low as 5 mph (2.2 m/s). Since most small turbines generate peak power at wind speeds of around 25 mph (11 m/s) and wind power follows the inverse cube law, I’d estimate that at 5 mph the 10 kW turbine generates only 80 Watts. (I asked the manufacturer to provide the numbers but they never responded.)

The much larger X3-20 (shown below) has all the features of the Mobile 50, and then some. It’s built into a standard sized shipping container that can be deployed by ship, train, or truck. It includes a heftier 12 kW wind turbine with a 30 foot self-deploying hydraulic mast, a 5 kW solar array (also expandable to 20 kW), a 48V bank of batteries, and a 48 hp (36 kW) generator. The generator options include gasoline, diesel, natural gas, or LP. The system can provide 20 kW of continuous power. The generator kicks on automatically when the renewables and batteries can’t provide enough power.

Once deployed, each system can be up and running in five minutes. X3 Energy estimates that the renewable energy sources can provide 70% of the total energy requirements, depending on local conditions, vastly reducing the amount of fuel required. The company plans to offer customized units for special applications.

Earlier I mentioned that this type of design has a lesson for off-grid designers. If you’re using renewables you’ll need some type of storage and for the average home consumer, that probably means batteries. How big of a battery bank do you need? We normally talk about “days of autonomy” - how many days you can run this system with little or no renewable sources available. For example, if your energy comes from solar panels, how many cloudy days can you endure without completely discharging your batteries? Depending on your location, you might expect two to five consecutive days with no sunshine. (PV panels generate some electricity on cloudy days, but nowhere near what they produce on sunny days.) A battery bank capable of holding five days’ worth of energy is very expensive. And although we call them “deep-cycle” batteries, discharging them more than 50% on a regular basis can drastically reduce their lives. (Battery technology is improving - especially for grid-level or microgrid storage - but that technology hasn’t reached the consumer level yet. So if you can only discharge a battery bank 50% and you need five days of autonomy, you’d effectively need a battery bank with ten days’ worth of storage. That’s way too expensive. A reasonable compromise is to use a smaller battery bank with about two days of autonomy along with an engine generator as a backup for longer stretches without sunlight. You can buy a generator and a lot of fuel for the cost of those extra batteries and if your system is well-sized and your building and appliances are energy-efficient, you wouldn’t be using the generator all that often.

The solution to our energy problems won’t come from a single technology, but a combination of several. There is strength in diversity, and these X3 Energy units demonstrate that.